Method and apparatus for carbonylating methanol with acetic acid enriched flash stream

ABSTRACT

A carbonylation process for producing acetic acid including: (a) carbonylating methanol or its reactive derivatives in the presence of a Group VIII metal catalyst and methyl iodide promoter to produce a liquid reaction mixture including acetic acid, water, methyl acetate and methyl iodide; (b) feeding the liquid reaction mixture at a feed temperature to a flash vessel which is maintained at a reduced pressure; (c) heating the flash vessel while concurrently flashing the reaction mixture to produce a crude product vapor stream, wherein the reaction mixture is selected and the flow rate of the reaction mixture fed to the flash vessel as well as the amount of heat supplied to the flash vessel is controlled such that the temperature of the crude product vapor stream is maintained at a temperature less than 90° F. cooler than the feed temperature of the liquid reaction mixture to the flasher and the concentration of acetic acid in the crude product vapor stream is greater than 70% by weight of the crude product vapor stream.

CROSS REFERENCE TO RELATED CASE

This application is a Divisional application of U.S. patent applicationSer. No. 12/150,481, entitled “Method and Apparatus for CarbonylatingMethanol With Acetic Acid Enriched Flash Stream”, filed Apr. 29, 2008,now U.S. Pat. No. ______. The priority of U.S. patent application Ser.No. 12/150,481 is hereby claimed and its disclosure incorporated hereinby reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to acetic acid manufacture with improvedefficiency provided by way of heating a flash vessel to maintain anelevated flash vapor temperature, generally more than 300° F. By way ofthe invention, the relative content of acetic acid in the crude productstream is increased, de-bottlenecking purification.

BACKGROUND

Acetic acid production by way of methanol carbonylation is well known inthe art. Generally speaking, a methanol carbonylation production lineincludes a reactor, a flasher, purification and recycle. In the reactorsection, methanol and carbon monoxide are contacted with rhodium oriridium catalyst in a homogenous stirred liquid phase reaction medium ina reactor to produce acetic acid. Methanol is pumped to the reactor froma methanol surge tank. The process is highly efficient, having aconversion of methanol to acetic acid of typically greater than 99percent. A flash vessel coupled to the reactor flashes a draw stream inorder to remove crude product from the reaction mixture. The crudeproduct is fed to a purification section which includes generally alight ends or stripper column, a drying column, auxiliary purificationand optionally a finishing column. In the process, various vent streamscontaining light ends, notably methyl iodide, carbon monoxide and methylacetate are generated and fed to a light ends recovery section. Thesevent streams are scrubbed with a solvent to remove the light ends whichare returned to the system or discarded.

It has been noted in various references that flash vessels used incarbonylation production processes may or may not be heated. See U.S.Pat. No. 5,874,610 to Clode et al. at Col. 2, lines 20-54; U.S. Pat. No.5,750,007 to Clode et al. at Col. 2, lines 40-51; and U.S. Pat. No.5,990,347 to Clode at Col. 2, lines 50-57. See also, U.S. Pat. No.6,066,762 to Yoneda et al. which discloses a flash temperature of from80° C.-180° C. (Col. 16, lines 40-44). It has not been appreciated,however, that temperature control within a relatively narrow window canbe used to greatly increase the acetic acid content of the crude productstream in an acetic acid process. In conventional systems, flashing istypically carried out adiabatically and there is a large temperaturedrop relative to the feed stream because of the heat of vaporization ofthe crude product.

SUMMARY OF THE INVENTION

It has been unexpectedly determined in accordance with the presentinvention that moderate heat input to the flasher vessel can greatlyincrease the concentration of acetic acid in the crude product stream,reducing purification and recycle requirements. This finding is notintuitively apparent to one of skill in the art. Without intending to bebound by theory, it is believed that elevated flash temperaturesvaporize more acetic acid and have little effect on the amount of lightends (methyl iodide, methyl acetate) that are flashed to the crudeproduct vapor stream.

There is thus provided in one aspect of the invention a carbonylationprocess for producing acetic acid comprising: (a) carbonylating methanolor its reactive derivatives in the presence of a Group VIII metalcatalyst and methyl iodide promoter to produce a liquid reaction mixtureincluding acetic acid, water, methyl acetate and methyl iodide; (b)feeding the liquid reaction mixture to a flash vessel which ismaintained at a reduced pressure; (c) heating the flash vessel whileconcurrently flashing the reaction mixture to produce a crude productvapor stream, wherein the reaction mixture is selected and the flow rateof the reaction mixture to the flash vessel as well as the amount ofheat supplied to the flash vessel is controlled such that thetemperature of the crude product vapor stream is maintained at atemperature of greater than 300° F. and the concentration of acetic acidin the crude product vapor stream is greater than 70% by weight of thestream.

Further details and advantages will become apparent from the discussionwhich follows.

BRIEF DESCRIPTION OF DRAWINGS

The invention is described in detail below with reference to thedrawings wherein like numerals designate similar parts. In the Figures:

FIG. 1 is a schematic diagram showing a methanol carbonylation apparatuswith purification;

FIG. 2 is a schematic diagram showing an alternate layout of the reactorand flasher vessels wherein there is provided a heat exchanger forproviding heat from the reactor to the flasher and a converter vesselbetween the reactor and flasher;

FIG. 3 is a flow chart schematically illustrating operation of theapparatus of FIGS. 1 and 2;

FIG. 4 is a graph showing crude product vapor concentration as afunction of flasher temperature;

FIG. 5 is a plot illustrating composition of the flash liquid vs.flasher temperature;

FIG. 6 is a plot of normalized mass flow rate of the various componentsin the flash vapor vs. flash temperature;

FIG. 7 is a plot of mass flow rates of various streams vs. flashtemperature; and

FIG. 8 is a plot illustrating heated flasher energy consumption and costvs. temperature.

DETAILED DESCRIPTION

The invention is described in detail below with reference to numerousembodiments for purposes of exemplification and illustration only.Modifications to particular embodiments within the spirit and scope ofthe present invention, set forth in the appended claims, will be readilyapparent to those of skill in the art.

Unless more specifically defined below, terminology as used herein isgiven its ordinary meaning. %, ppm and like terms refer to weightpercent and parts per million by weight, unless otherwise indicated.

“Reduced pressure” refers to a pressure less than that of the reactorvessel.

A “like” stream undergoing flashing refers to a feed stream of the samecomposition which yields a product stream having the same flow rate ofacetic acid in the flash vapor. See Tables 1-7.

The feed temperature of the reaction mixture to the flasher is measuredas close as practical to the inlet of the flasher, on the high pressureside. Any suitable instrumentation may be used.

The temperature of the crude product vapor stream is measured as closeas practical to the vapor outlet of the flasher vessel.

A Group VIII catalyst metal used in connection with the presentinvention may be a rhodium and/or iridium catalyst. The rhodium metalcatalyst may be added in any suitable form such that rhodium is in thecatalyst solution as an equilibrium mixture including [Rh(CO)₂I₂]⁻ anionas is well known in the art. When rhodium solution is in the carbonmonoxide-rich environment of the reactor, solubility of the rhodium isgenerally maintained because rhodium/carbonyl iodide anionic species aregenerally soluble in water and acetic acid. However, when transferred tocarbon monoxide depleted environments as typically exist in the flasher,light ends column and so forth, the equilibrium rhodium/catalystcomposition changes since less carbon monoxide is available. Rhodiumprecipitates as RhI₃, for example; details as to the form of entrainedrhodium downstream of the reactor are not well understood. Iodide saltshelp alleviate precipitation in the flasher under so-called “low water”conditions as will be appreciated by one of skill in the art.

Iodide salts maintained in the reaction mixtures of the processesdescribed herein may be in the form of a soluble salt of an alkali metalor alkaline earth metal or a quaternary ammonium or phosphonium salt. Incertain embodiments, the catalyst co-promoter is lithium iodide, lithiumacetate, or mixtures thereof. The salt co-promoter may be added as anon-iodide salt or ligand that will generate an iodide salt. The iodidecatalyst stabilizer may be introduced directly into the reaction system.Alternatively, the iodide salt may be generated in-situ since under theoperating conditions of the reaction system, a wide range of non-iodidesalt precursors will react with methyl iodide to generate thecorresponding co-promoter iodide salt stabilizer. For additional detailregarding iodide salt generation, see U.S. Pat. Nos. 5,001,259 to Smithet al.; 5,026,908 to Smith et al.; and 5,144,068, also to Smith et al.,the disclosures of which are hereby incorporated by reference. Theiodide salt may be added as a phosphine oxide or any organic ligand, ifso desired. These compounds and other ligands generally undergoquaternization in the presence of methyl iodide at elevated temperaturesto yield suitable salts which maintain iodide anion concentration.

An iridium catalyst in the liquid carbonylation reaction composition maycomprise any iridium-containing compound which is soluble in the liquidreaction composition. The iridium catalyst may be added to the liquidreaction composition for the carbonylation reaction in any suitable formwhich dissolves in the liquid reaction composition or is convertible toa soluble form. Examples of suitable iridium-containing compounds whichmay be added to the liquid reaction composition include: IrCl₃, IrI₃,IrBr₃, [Ir(CO)₂I]₂, [Ir(CO)₂Cl]₂, [Ir(CO)₂Br]₂, [Ir(CO)₂I₂]⁻H⁺,[Ir(CO)₂Br₂]⁻H⁺, [Ir(CO)₂I₄]⁻H⁺, [Ir(CH₃)I₃(CO)₂]⁻H⁺, Ir₄(CO)₁₂,IrCl₃.3H₂O, IrBr₃.3H₂O, Ir₄(CO)₁₂, iridium metal, Ir₂O₃, Ir(acac)(CO)₂,Ir(acac)₃, iridium acetate, [Ir₃O(OAc)₆(H₂O)₃][OAc], andhexachloroiridic acid [H₂IrCl₆]. Chloride-free complexes of iridium suchas acetates, oxalates and acetoacetates are usually employed as startingmaterials. The iridium catalyst concentration in the liquid reactioncomposition may be in the range of 100 to 6000 ppm. The carbonylation ofmethanol utilizing iridium catalyst is well known and is generallydescribed in the following U.S. Pat. Nos. 5,942,460; 5,932,764;5,883,295; 5,877,348; 5,877,347 and 5,696,284, the disclosures of whichare hereby incorporated by reference into this application as if setforth in their entirety.

Methyl iodide is used as the promoter. Preferably, the concentration ofmethyl in the liquid reaction composition is in the range 1 to 50% byweight, preferably 2 to 30% by weight.

The promoter may be combined with a salt stabilizer/co-promotercompound, which may include salts of a metal of Group IA or Group IIA,or a quaternary ammonium or phosphonium salt. Particularly preferred areiodide or acetate salts, e.g., lithium iodide or lithium acetate.

Other promoters and co-promoters may be used as part of the catalyticsystem of the present invention as described in European PatentPublication EP 0 849 248, the disclosure of which is hereby incorporatedby reference. Suitable promoters are selected from ruthenium, osmium,tungsten, rhenium, zinc, cadmium, indium, gallium, mercury, nickel,platinum, vanadium, titanium, copper, aluminum, tin, antimony, and aremore preferably selected from ruthenium and osmium. Specificco-promoters are described in U.S. Pat. No. 6,627,770, the entirety ofwhich is incorporated herein by reference.

A promoter may be present in an effective amount up to the limit of itssolubility in the liquid reaction composition and/or any liquid processstreams recycled to the carbonylation reactor from the acetic acidrecovery stage. When used, the promoter is suitably present in theliquid reaction composition at a molar ratio of promoter to metalcatalyst of [0.5 to 15]:1, preferably [2 to 10]:1, more preferably [2 to7.5]:1. A suitable promoter concentration is 400 to 5000 ppm.

The present invention may be appreciated in connection with, forexample, the carbonylation of methanol with carbon monoxide in ahomogeneous catalytic reaction system comprising a reaction solvent(typically acetic acid), methanol and/or its reactive derivatives, asoluble rhodium catalyst, and at least a finite concentration of water.The carbonylation reaction proceeds as methanol and carbon monoxide arecontinuously fed to the reactor. The carbon monoxide reactant may beessentially pure or may contain inert impurities such as carbon dioxide,methane, nitrogen, noble gases, water and C₁ to C₄ paraffinichydrocarbons. The presence of hydrogen in the carbon monoxide andgenerated in situ by the water gas shift reaction is preferably keptlow, for example, less than 1 bar partial pressure, as its presence mayresult in the formation of hydrogenation products. The partial pressureof carbon monoxide in the reaction is suitably in the range 1 to 70 bar,preferably 1 to 35 bar, and most preferably 1 to 15 bar.

The pressure of the carbonylation reaction is suitably in the range 10to 200 bar, preferably 10 to 100 bar, most preferably 15 to 50 bar. Thetemperature of the carbonylation reaction is suitably in the range 100to 300° C., preferably in the range 150 to 220° C. Acetic acid istypically manufactured in a liquid phase reaction at a temperature offrom about 150-200° C. and a total pressure of from about 20 to about 50bar.

Acetic acid is typically included in the reaction mixture as the solventfor the reaction.

Suitable reactive derivatives of methanol include methyl acetate,dimethyl ether, methyl formate and methyl iodide. A mixture of methanoland reactive derivatives thereof may be used as reactants in the processof the present invention. Preferably, methanol and/or methyl acetate areused as reactants. At least some of the methanol and/or reactivederivative thereof will be converted to, and hence present as, methylacetate in the liquid reaction composition by reaction with acetic acidproduct or solvent. The concentration in the liquid reaction compositionof methyl acetate is suitably in the range 0.5 to 70% by weight,preferably 0.5 to 50% by weight, more preferably 1 to 35% by weight andmost preferably 1-20% by weight.

Water may be formed in situ in the liquid reaction composition, forexample, by the esterification reaction between methanol reactant andacetic acid product. Water may be introduced to the carbonylationreactor together with or separately from other components of the liquidreaction composition. Water may be separated from other components ofreaction composition withdrawn from the reactor and may be recycled incontrolled amounts to maintain the required concentration of water inthe liquid reaction composition. Preferably, the concentration of watermaintained in the liquid reaction composition is in the range 0.1 to 16%by weight, more preferably 1 to 14% by weight, most preferably 1 to 10%by weight.

The reaction liquid is typically drawn from the reactor and flashed in aone step or multi-step process using a converter as well as a flashvessel as hereinafter described. The crude vapor process stream from theflasher is sent to a purification system which generally includes atleast a light ends column and a dehydration column.

The present invention is further appreciated by reference to FIG. 1which is a schematic diagram illustrating a typical carbonylationprocess and apparatus. In FIG. 1 there is shown a carbonylation system10 including a reactor 12 provided with a feed system 14 including amethanol surge tank 16 and carbon monoxide feed line 18. A catalystreservoir system includes a methyl iodide storage vessel 20 as well as acatalyst storage tank 22. Reactor 12 is provided with a vent 24 and anoptional vent 24 a. Reactor 12 is coupled to a flash vessel 26 by way ofa conduit 28 and optionally by way of vent 24 a. The flasher, in turn,is coupled to a purification section 30 which includes a light ends orstripper column 32, a dehydration column 34 and a strong acid,silver-exchanged cation ion-exchange resin bed 36 which removes iodidesfrom the product. Instead of a silver-exchanged, strong acid cationion-exchange resin, it has been reported that anion ion-exchange resincan be used to remove iodides. See British Patent No. G 2112394A, aswell as U.S. Pat. No. 5,416,237, Col. 7, lines 54+, which teaches theuse of 4-vinylpyridine resins for iodide removal.

A gaseous purge stream is typically vented from the head of the reactorto prevent buildup of gaseous by-products such as methane, carbondioxide and hydrogen and to maintain a set carbon monoxide partialpressure at a given total reactor pressure. Optionally (as illustratedin Chinese Application No. ZL92108244.4, published as Chinese Patent No.CN1069262 A), a so-called “converter” reactor can be employed which islocated between the reactor and flasher vessel shown in FIG. 1 anddiscussed further in connection with FIG. 2. Optionally, the gaseouspurge streams may be vented through the flasher base liquid or lowerpart of the light ends column to enhance rhodium stability and/or theymay be combined with other gaseous process vents (such as thepurification column overhead receiver vents) prior to scrubbing. Thesevariations are well within the scope of the present invention as will beappreciated from the appended claims and the description which follows.

As will be appreciated by one of skill in the art, the differentchemical environments encountered in the purification train may requiredifferent metallurgy. For example, equipment at the outlet of the lightends column will likely require a zirconium vessel due to the corrosivenature of the process stream, while a vessel of stainless steel may besufficient for equipment placed downstream of the dehydration columnwhere conditions are much less corrosive.

Carbon monoxide and methanol are introduced continuously into reactor 12with adequate mixing at a high carbon monoxide partial pressure. Thenon-condensable by-products are vented from the reactor to maintain anoptimum carbon monoxide partial pressure. The reactor off gas is treatedto recover reactor condensables, i.e., methyl iodide before flaring.Methanol and carbon monoxide efficiencies are generally greater thanabout 98 and 90% respectively. As will be appreciated from the Smith etal. patent noted above, major inefficiencies of the process are theconcurrent manufacture of carbon dioxide and hydrogen by way of thewater gas shift reaction.

From the reactor, a stream of the reaction mixture is continuously fedvia conduit 28 to flasher 26. Through the flasher the product aceticacid and the majority of the light ends (methyl iodide, methyl acetate,and water) are separated from the reactor catalyst solution, and thecrude process stream 38 is forwarded with dissolved gases to thedistillation or purification section 30 in single stage flash. Thecatalyst solution is recycled to the reactor via conduit 40. Inaccordance with the invention, the flasher is heated with steam, forexample, by way of jacketing or coils in order to raise the temperatureof stream 38. Alternative heating means such as electric heating orradiant (microwave) heating can be used if more convenient.

The purification of the acetic acid typically includes distillation in alight ends column, a dehydration column, and, optionally, a heavy endscolumn. The crude vapor process stream 38 from the flasher is fed intothe light ends column 32. Methyl iodide, methyl acetate, and a portionof the water condense overhead in the light end columns to form twophases (organic and aqueous) in a receiver 42. Both overhead liquidphases return to the reaction section via recycle line 44. Optionally, aliquid recycle stream 45 from the light ends column may also be returnedto the reactor.

The purified process stream 50 is drawn off the side of the light endscolumn 32 and is fed into dehydration column 34. Water and some aceticacid from this column separate and are recycled to the reaction systemvia recycle line 44 as shown. The purified and dried process stream 52from the dehydration column 34 feeds resin bed 36 and product is takentherefrom at 56 as shown. Carbonylation system 10 uses only two primarypurification columns and is preferably operated as described in moredetail in U.S. Pat. No. 6,657,078 to Scates et al., entitled “Low EnergyCarbonylation Process”, the disclosure of which is incorporated hereinby reference. Additional columns are generally used as desired,depending on the system.

There is shown in FIG. 2 an alternate layout of the reactor/flasher witha converter vessel 12 a therebetween as well as a heat exchanger 60 anda low pressure steam flash vessel 62. Reactor 12 and flasher 26 operateas described above. Methanol and carbon monoxide are provided to reactor12 at 18 a, 18 and liquid reaction mixture is drawn at 28 a and providedto converter vessel 12 a which vents gas including light ends to ascrubber (not shown). The vent gas can be scrubbed with methanol andreturned to the reactor. Converter 12 a feeds flasher 26 where thepressure is reduced and flashed to crude product stream 38. Recycle tothe reactor is provided by way of lines 40, 44 as is discussed above inconnection with FIG. 1.

Flasher 26 is heated by way of a low pressure steam supply 64 providedfrom a steam flash vessel 62 which is fed from heat exchanger 60. Heatexchanger 60 is made with suitable metallurgy and receives hot catalyticmixture from reactor 12 via line 66 as well as steam condensate via line68. The condensate is heated by the hot catalyst which, in turn,requires cooling because of the exothermic nature of the carbonylationreaction. The heated condensate is supplied to vessel 62 via line 70where it is flashed to (low pressure) steam and used to heat flasher 26as noted above.

Thus, heat exchanger 60 as shown in FIG. 2 provides cooling to thereactor and heat to the flasher which reduces overall energy costs aswill be appreciated by one of skill in the art.

Carbon monoxide may be added directly to converter 12 a if so desired ormay be added slightly before (upstream) or after (downstream) if sodesired in order to stabilize the catalyst solution and consume anyunreacted methanol. Details of such arrangements are seen in EuropeanPatent No. EP 0 759 419 as well as U.S. Pat. No. 5,770,768 to Denis etal., the disclosures of which are hereby incorporated by reference.

Whether or not heat transfer from the reactor to the flasher isemployed, the present invention substantially increases the efficiencyof the system by providing a higher concentration of acetic acid in thecrude product vapor stream as will be appreciated from the discussionwhich follows.

The carbonylation apparatus shown in FIG. 1 and that illustrated in FIG.2 can be represented schematically as shown in FIG. 3 for presentpurposes. In FIG. 3, the feed to the reactor is designated stream 1, theliquid stream to the flasher is designated stream 2, the crude productvapor stream provided to the splitter column is designated stream 3 andthe purified product stream is labeled stream 4. Stream 5 represents thecatalyst recycle stream from the flasher and stream 6 represents recyclefrom purification recycle to the reactor.

FIG. 3 illustrates two major inefficiencies of the methanolcarbonylation process generally; catalyst recycle (5) and purificationrecycle (6). Both of these internal ‘flywheels’ are energy andcapital-intensive and could be minimized by improving performance of theflasher—by ensuring that the vapor stream that it sends to purification(3) has proportionally more HAc and less “non-product” components (H₂O,MeAc, MeI). This can be accomplished by providing heat input to raisethe operating temperature of the flasher. The benefits of this conceptare illustrated in the following examples.

A semi-empirical simulator was used to study the effect of flashtemperature while holding constant the mass flow of HAc in the vaporstream (3). The stream compositions are shown below for vapor (3) andliquid (5) exiting the flasher. The flasher inlet basis is a stream at387° P, 400 psig, containing 8.1 wt % MeI, 2.9 wt % MeAc, 75.7 wt % HAc,2.8 wt % H₂O, and 10.6 wt % LiI. Flash temperature (temperature of thevapor stream) was varied from adiabatic (297° F.) to isothermal (387°F.), all cases to 25 psig.

Results appear in Tables 1-7 and FIGS. 4-7.

TABLE 1 Comparative Example A - Adiabatic Operation of Flasher INLETVAPOR LIQUID T (F.) 387 297 297 Flow Total 1554.42 260.37 1294.05Total-HAc 89.48 MeI 125.17 58.31 66.86 MeAc 45.18 20.42 24.76 HAc1175.98 170.89 1005.09 H₂O 43.81 10.76 33.05 LiI 164.29 0.00 164.29Weight % MeI 8.1 22.4 5.2 MeAc 2.9 7.8 1.9 HAc 75.7 65.6 77.7 H₂O 2.84.1 2.6 LiI 10.6 0.0 12.7

TABLE 2 Example 1 - Operation of Flasher Maintaining Vapor at 300° F.INLET VAPOR LIQUID T (F.) 387 300 300 Flow Total 878.28 241.52 636.76Total-HAc 70.63 MeI 70.72 44.96 25.76 MeAc 25.53 15.82 9.70 HAc 664.45170.89 493.56 H₂O 24.75 9.84 14.91 LiI 92.82 0.00 92.82 Weight % MeI 8.118.6 4.0 MeAc 2.9 6.6 1.5 HAc 75.7 70.8 77.5 H₂O 2.8 4.1 2.3 LiI 10.60.0 14.6

TABLE 3 Example 2 - Operation of Flasher Maintaining Vapor at 305° F.INLET VAPOR LIQUID T (F.) 387 305 305 Flow Total 480.70 222.04 258.66Total-HAc 51.15 MeI 38.71 31.48 7.23 MeAc 13.97 11.17 2.80 HAc 363.67170.89 192.78 H₂O 13.55 8.50 5.04 LiI 50.80 0.00 50.80 Weight % MeI 8.114.2 2.8 MeAc 2.9 5.0 1.1 HAc 75.7 77.0 74.5 H₂O 2.8 3.8 1.9 LiI 10.60.0 19.6

TABLE 4 Example 3 - Operation of Flasher Maintaining Vapor at 310° F.INLET VAPOR LIQUID T (F.) 387 310 310 Flow Total 351.38 212.96 138.42Total-HAc 42.07 MeI 28.29 25.35 2.94 MeAc 10.21 9.05 1.16 HAc 265.83170.89 94.94 H₂O 9.90 7.67 2.24 LiI 37.14 0.00 37.14 Weight % MeI 8.111.9 2.1 MeAc 2.9 4.2 0.8 HAc 75.7 80.2 68.6 H₂O 2.8 3.6 1.6 LiI 10.60.0 26.8

TABLE 5 Example 4 - Operation of Flasher Maintaining Vapor at 325° F.INLET VAPOR LIQUID T (F.) 387 325 325 Flow Total 265.59 205.71 59.88Total-HAc 34.82 MeI 21.39 20.58 0.81 MeAc 7.72 7.39 0.33 HAc 200.93170.89 30.04 H₂O 7.49 6.86 0.63 LiI 28.07 0.00 28.07 Weight % MeI 8.110.0 1.4 MeAc 2.9 3.6 0.6 HAc 75.7 83.1 50.2 H₂O 2.8 3.3 1.0 LiI 10.60.0 46.9

TABLE 6 Example 5 - Operation of Flasher Maintaining Vapor at 350° F.INLET VAPOR LIQUID T (F.) 387 350 350 Flow Total 241.97 203.50 38.47Total-HAc 32.61 MeI 19.48 19.14 0.34 MeAc 7.03 6.89 0.14 HAc 183.06170.89 12.17 H₂O 6.82 6.58 0.24 LiI 25.57 0.00 25.57 Weight % MeI 8.19.4 0.9 MeAc 2.9 3.4 0.4 HAc 75.7 84.0 31.6 H₂O 2.8 3.2 0.6 LiI 10.6 0.066.5

TABLE 7 Example 6 - Isothermal Operational Flasher INLET VAPOR LIQUID T(F.) 387 387 387 Flow Total 233.35 202.68 30.67 Total-HAc 31.79 MeI18.79 18.61 0.18 MeAc 6.78 6.71 0.08 HAc 176.54 170.89 5.64 H₂O 6.586.47 0.11 LiI 24.66 0.00 24.66 Weight % MeI 8.1 9.2 0.6 MeAc 2.9 3.3 0.2HAc 75.7 84.3 18.4 H₂O 2.8 3.2 0.3 LiI 10.6 0.0 80.4As shown in the data and on FIG. 4, increasing the flasher temperatureincreases the HAc wt % in the vapor stream (3) while decreasingconcentrations of all other components. FIG. 5 illustrates that theproportion of LiI in the catalyst recycle stream (5) increases withincreasing flash temperature. This high LiI acts to improve catalyststability in the flasher (possibly compensating for any detrimentaleffects of higher operating temperature).

FIG. 6 shows the effect of flasher temperature on the mass flow rate ofeach component in the vapor stream that is fed to purification (3). Itshows that for a set amount of HAc throughput, smaller quantities of the“non-product” components are sent to purification when using a higherflash temperature. For example, raising the flash temperature from 297to 310° F. would decrease the mass flow of water sent to purification by30%, MeAc by 55% and MeI by 55%.

It is seen in FIG. 7 that the flow rate requirements of the streams aresignificantly lower when operating the flasher at a higher temperature.This is a result of proportionally more HAc in the vapor stream exitingthe flasher (3) and less of the “non-product” components. A lower flowrate of flasher feed (2) is required to attain the same mass throughputof HAc to purification (3). For example, by raising flash temperaturefrom 297 to 310° F., the required catalyst recycle rate drops by 90%,liquid feed to flasher by 80%, purification recycle by 50% and vaporfeed to purification by 20%. Benefits include: (1) for an existing unit,increasing HAc in the crude product stream, thus debottleneckingpurification and lower operating costs and/or allow an increase incapacity; (2) running the reactor at higher MeAc (currently this levelis typically constrained by purification capacity; higher MeAc alsoallows the reactor to operate at a lower temperature and also decreasesthe make rate of propionic acid); (3) for a new unit, reducing thecapital and energy requirements by requiring less catalyst recycle andpurification throughput for a given production rate of HAc; (4)decreasing vapor feed rate to purification which reduces catalyst lossvia entrainment; and (5) decreasing liquid feed rate to the flasherwhich improves CO efficiency by significantly reducing the carryoverloss of soluble CO (which currently accounts for 80% of the total COwaste).

For example, increasing flasher operating temperature from 297 to 310°decreases the required flowrate to the flasher by 80%. This modificationdecreases the total CO inefficiency dramatically, by −60% (=80%reduction of the 80% of CO loss from flasher carryover).

The energy cost of heating the flasher with steam is shown in FIG. 8.This cost would be significantly reduced by integrating heat between thereactor and flasher as is shown in FIG. 2. For example, to heat to 310°F., it is possible to use the reactor cooling loop to heat the flasher.

While the invention has been illustrated in connection with particularequipment and operating conditions, modifications to these exampleswithin the spirit and scope of the invention will be readily apparent tothose of skill in the art. In view of the foregoing discussion, relevantknowledge in the art and references discussed above in connection withthe Background and Detailed Description, the disclosures of which areall incorporated herein by reference, further description is deemedunnecessary.

1. A carbonylation process for producing acetic acid comprising: (a)carbonylating methanol or its reactive derivatives in the presence of aGroup VIII metal catalyst and methyl iodide promoter to produce a liquidreaction mixture including acetic acid, water, methyl acetate and methyliodide; (b) feeding the liquid reaction mixture at a feed temperature toa flash vessel which is maintained at a reduced pressure; (c) heatingthe flash vessel while concurrently flashing the reaction mixture toproduce a crude product vapor stream, wherein the reaction mixture isselected and the flow rate of the reaction mixture to the flash vesselas well as the amount of heat supplied to the flash vessel is controlledsuch that the temperature of the crude product vapor stream ismaintained at a temperature less than 90° F. cooler than the feedtemperature of the liquid reaction mixture fed to the flasher and theconcentration of acetic acid in the crude product vapor stream isgreater than 70% by weight of the crude product vapor stream.
 2. Thecarbonylation process according to claim 1, wherein the reaction mixtureis selected and its flow rate controlled along with the heat supplied tothe flasher such that the crude product stream has an acetic acidconcentration of at least 75 weight % of the stream.
 3. Thecarbonylation process according to claim 1, wherein the reaction mixtureis selected and its flow rate controlled along with the heat supplied tothe flasher such that the crude product stream has an acetic acidconcentration of at least 80 weight % of the stream.
 4. Thecarbonylation process according to claim 1, wherein the reaction mixtureis selected and its flow rate controlled along with the heat supplied tothe flasher such that the crude product stream has an acetic acidconcentration of from 80 weight % to 85 weight % of the stream.
 5. Thecarbonylation process according to claim 1, wherein the crude productvapor stream has a temperature less than 85° F. cooler than thetemperature of the liquid reaction mixture stream fed to the flasher. 6.The carbonylation process according to claim 1, wherein the crudeproduct vapor stream has a temperature less than 80° F. cooler than thetemperature of the liquid reaction mixture stream fed to the flasher. 7.The carbonylation process according to claim 1, wherein the crudeproduct vapor stream has a temperature less than 75° F. cooler than thetemperature of the liquid reaction mixture stream fed to the flasher. 8.The carbonylation process according to claim 1, wherein the crudeproduct vapor stream has a temperature less than 65° F. cooler than thetemperature of the liquid reaction mixture stream fed to the flasher. 9.The carbonylation process according to claim 1, wherein the crudeproduct vapor stream has a temperature less than 60° F. cooler than thetemperature of the liquid reaction mixture stream fed to the flasher.10. An apparatus for producing acetic acid comprising: (a) a reactor forcarbonylating methanol or its reactive derivatives in the presence of aGroup VIII metal catalyst and methyl iodide promoter to produce a liquidreaction mixture including acetic acid, water, methyl acetate and methyliodide; (b) a flash vessel adapted to receive a stream of the reactionmixture and flash the reaction mixture at a reduced pressure to producea crude product vapor stream; and (c) a heat transfer system coupled tothe reactor and the flash vessel operative to transfer heat from thereactor to the flash vessel so as to elevate the temperature of thecrude product vapor stream as compared with the temperature of a likestream undergoing adiabatic flashing.
 11. The apparatus according toclaim 10, further comprising a converter vessel coupled to the reactorand the flasher.
 12. The apparatus according to claim 10, furthercomprising a splitter column adapted for receiving the crude productstream and removing methyl acetate and methyl iodide therefrom toproduce a purified product stream.
 13. The apparatus according to claim12, further comprising a drying column adapted for receiving thepurified product stream from the splitter column and removing watertherefrom.